II vs couplet

II and couplet both are nouns.

Word	Noun	Adjective	Verb	Adverb
II	Yes	No	No	No
couplet	Yes	No	No	No

As nouns, couplet is a hyponym of II; that is, couplet is a word with a more specific, narrower meaning than II:

II: the cardinal number that is the sum of one and one or a numeral representing this number
couplet: two items of the same kind

Other hyponyms of II include craps, snake eyes, brace, couple, distich, duad, duet, duo, dyad, pair, span, twain, twosome, yoke.

II (noun)	couplet (noun)
the cardinal number that is the sum of one and one or a numeral representing this number	a stanza consisting of two successive lines of verse; usually rhymed
	two items of the same kind